Sigma-delta converters are used, for example, for converting an analog signal into a digital data stream. For this purpose, the input signal of the sigma-delta converter is modulated in accordance with the customary sigma-delta modulation principle by means of one or more modulation stages and is fed to a quantizer. In the quantizer, the modulated signal is sampled with two or more quantization stages and quantized such that a quantized data stream with a certain word size is created, wherein this word size depends on the number of quantization stages. Consequently, a binary bit stream is created if only two quantization stages are provided.
In conventional sigma-delta converters, the modulated signal or the modulator can become unstable if the input signal level exceeds certain limits. Consequently, the sigma-delta converter cannot be used in input signal ranges that could theoretically be represented by the digital quantized signal. For example, a sigma-delta converter with a sigma-delta modulator of the fourth order and a one bit output would be unable to process input signals that would produce a modulated signal with a modulation rate greater than ±70%. Expressed in digital terms, the modulation rate may, for example, result as the ratio of the numerical difference between bits with high signal level and bits with low signal level referred to the overall number of bits observed. The closer the input signal level lies to a theoretical positive or negative limit of the input signal range, the higher the amount of the corresponding theoretical modulation rate becomes.
In order to solve these instability problems, it is possible, for example, to adapt the input amplitude of the signal to be converted in such a way that the limited input signal range is no longer relevant and this input signal adaptation is compensated by once again raising the level of the digital quantized signal, for example, by an amplification factor of a downstream digital filter.
However, if such a digital filter cannot be influenced, for example, because the amplification factor is fixed, conventional sigma-delta converters do not make it possible to utilize a theoretically possible input signal range and to simultaneously ensure the stability of the sigma-delta converter.